PURPOSE: To enable the measuring instrument to detect the flow rate of air with higher accuracy by providing an air breeding pass for correcting the flow of the air in the passage of an exothermic resistor.

CONSTITUTION: The body 1 of the title measuring instrument is formed by die-casting or treating a plastic, etc. A by-pass 3 is provided to part of a main pass 2 in the body 1 and an air breeding pass 7 is provided from the by-pass 3. An exothermic resistor 4 is set in the by-pass 3 and a temperature sensitive resistor 6 for detecting air temperature is similarly set on the downstream of the resistor 4. The resistor 4 detects the flow rate fluctuation of air as the fluctuation of an electric current consumed for heat generation with high accuracy through a circuit module 5. The flow rate of air is measured by using the resistor 4 and the flow rate is balanced in a whole flowing area by providing the pass 7 in the by-pass 3. Namely, by providing an air breeding throttle 8 in the pass 7, the flow rate of the air is adjusted so as to suppress the pulsation of the intake air in the whole flowing area.